1. Field of the Invention
This invention relates to a digital image signal processing technique, and more particularly to a technique of correcting a color tone of a color display screen of a computer apparatus, a television screen, a copying machine, a photograph, a print, a color facsimile and other apparatus by which a color image is displayed. More specifically, the present invention relates to an apparatus which can correct, when to correct a particular color tone displayed on a screen, the particular color tone displayed on the screen independently or can correct not only the particular color tone but also other color tones than the particular color tone in an associated relationship.
2. Description of the Related Art
In the first of color printing, a technique of correcting or modifying a finished color tone is conventionally known. A skilled person visually observes a color page of a trial print to discriminate whether or not the color tone of each part coincides with the color tone of an original or coincides with the color tone of an actual imaged object, and effects correction of a color mixture ratio for some color tone to effect desired printing.
Meanwhile, a technique of performing color tone processing using digital signal processing has been developed. According to the technique which involves digital signal processing, correction of various color tone offsets can be performed by calculation processing of signals (refer to Japanese Patent Laid-Open Application No. Heisei 4-334267).
As one of image correction apparatus of the type mentioned, a color correction apparatus for 6 different colors is known. A color correction apparatus for 6 different colors is disclosed in Kaji, "Printing Image Engineering", Printing Division of the Printing Engineering Society of Japan, 1988, pp. 376-379 and Tajima, "Theory of Color Image Reproduction", Maruzen, 1996, pp. 71-74, and so forth, and is mounted as a color modification apparatus for a scanner in a printing apparatus or a like apparatus. With the color correction apparatus for 6 different colors, a user selects a hue to be corrected from among six hues of R, G, B, C, M and Y (red, green, blue, cyani magenta and yellow) and corrects the colors of only pixels which belong to the designated hue. For correction of CMYK (cyan, magenta, yellow and black) data, the following correction expression: ##EQU1##
is used. Meanwhile, where the system described is applied to RGB data, the following correction expression: ##EQU2##
is used.
In the expressions above, dC, dM, dY, dR, dG and dB represent color components of cyan, magenta, yellow, red, green and blue of pixels in an original image, respectively. The values of them are hereinafter referred to as color approximation degrees of the color components. Further, the coefficients b11 to b46 and c11 to c36 are correction coefficients for different hues and are adjusted in an interactive fashion.
FIG. 9 is a diagrammatic view illustrating an RGB space projected on a plane perpendicular to an axis of white and black components. This projection plane is divided into specific color areas of 1 to 6. In other words, except gray components, all pixel values belong to the specific color areas of 1 to 1.
A method of calculating color approximation degrees (dR, dG, dB, dC, dM, dY) of the six hues from RGB values of pixels is illustrated in Table 1 below.
TABLE 1 Color Area Condition dR dG dB dC dM dV 1 R &gt; G &gt; B R - G 0 0 0 0 G - B 2 G &gt; R &gt; B 0 G - R 0 0 0 R - B 3 G &gt; B &gt; R 0 G - B 0 B - R 0 0 4 B &gt; G &gt; R 0 0 B - G G - R 0 0 5 B &gt; R &gt; G 0 0 B - R 0 R - G 0 6 R &gt; B &gt; G R - B 0 0 0 B - G 0
The conventional apparatus realizes such calculation using an IC. Further, a distribution in color approximation degree dR of R is illustrated in FIG. 9. It can be seen from FIG. 9 that pixels having R components are present in color approximation areas 1 to 6.
In the conventional apparatus, hues which can be designated are fixed to the predetermined six hues, and correction for intermediate hues or only for a color having a low saturation is impossible. For example, if an R hue is designated, then the range in which correction is possible includes a hue range (.+-.60 degrees) designated in FIG. 9. Accordingly, if it is tried to correct a particular hue, also patterns of hues of a same color family are corrected simultaneously.
On the other hand, in color printing, much time is required for hue correction. In particular, a skilled operator performs correction while performing complicated manual operations based on its abundant experience. A skilled person can effect hue correction finally as desired. However, a person who does not have abundant experience cannot perform such manual operations. Further, in many cases, manual operations which require such long time cannot be performed.
For example, as a technique which has spread widely in recent years, a technique wherein a person who does not have sufficient experience observes an output screen of a computer and manually operates a mouse to designate a desired region of an image so that the hue of the region is replaced simply with another hue is available.
Such a technique as just described is employed widely in various types of occupation which involve designing and can be utilized also for production of a home page when a user of a computer tries to provide a home page on the Internet. It can be considered that almost all of such users have little technical knowledge of color correction, and it is impossible for them to perform a complicated operation based on abundant experience as described above.
Further, where the technique described above is used in such types of occupation which involve designing, such a manner of use that the user effects simulation while observing a color image of a computer in front of an ordering person of the design and discusses the design with the ordering person of the design. In such an instance, it is required to replace a hue with another hue at a moment, and an apparatus which involves operations which require much time is not suitable for the application just described.